A biochemical and functional approach is being used to analyze the antigenic similarities and differences between the 17XL and 17XNL strains of Plasmodium yoelii. Over 125 malarial polypeptides have been identified using isoelectric focusing and SDS-PAGE. It appears that both quantitative and qualitative differences exist between the two strains. To date, twelve hybridoma cell lines, which secrete antiplasmodial antibodies (Ab), have been established. Monoclonal Ab are being used to identify and isolate important antigens of the erythrocytic stages of the parasite. In addition, polypeptides of P. yoelii are being separated by preparatory electrophoresis and fractions which contain antigens that react with hyperimmune serum and induce an in vitro proliferation response of sensitized, but not normal, lymphocytes have been identified.